Relative power measurements (i.e. obtaining the ratio of the power value of two signals) are required in many applications. For example, in network analysis the relative value of the input and output signals is a measure of circuit performance. It is inconvenient to manually record and subtract the two values expressed in decibels (dB) to obtain a relative power measurement, particularly when a great many such measurements must be taken. Also, there is a possibility of error in the recording and the arithmetic if manual methods are used.
Some previous power meters have provided relative power measurements using analog techniques. For example, some have used analog logarithmic converters with their inherent inaccuracies while others have used complex analog ratio techniques.
The preferred embodiment of the present invention provides accurate relative power measurements by first converting the logarithm of the measured power values to digital form. A power measurement is selected to be the reference measurement and its value is stored in a register. Subsequent power measurement values are stored in a second register. These power values are digitally processed to provide the relative power measurement.
In the preferred embodiment, the relative value is provided in dB by first converting the power measurement values to their base 10 logarithms. The logarithm is taken relative to a fixed base such as 1 milliwatt (mW). Obtaining the difference between the two logarithms yields the ratio, in decibels, of the two power measurements. This is more clearly shown by the following equations. ##EQU1##
P.sub.F is selected to be 1 mW in the preferred embodiment but may be any arbitrary value since it cancels out in the calculations, as shown above.
The conversion of an input signal to logarithmic form is fully explained in applicant's copending application entitled "Converter Circuit", Ser. No. 422,498, filed on Dec. 6, 1973, now U.S. Pat. No. 3,879,668.